Thunking describes any variety of process by which a first process executing a first set of machine executable instructions compiled for a first platform type, word size, etc. (e.g., 32-bit code) is able to successfully make a function call to another set of machine executable instructions compiled for a second platform type, word size, etc. (e.g., 16-bit code). In general, thunking involves a translation of function calls, function call parameters and/or return parameters between at least two processes compiled for different platform types, word sizes, etc.
For instance, consider an example in which a process executing native code compiled for a particular type of processor makes a function call to a platform-independent byte code function being executed by a virtual machine that is also executed upon the processor. Thunking for such an example system involves having the virtual machine copy data (e.g., calling parameters) from the processor's registers and/or slots of the native code's run-time stack (i.e., native stack slots) into a byte code stack before the byte code is executed by the virtual machine. The copying of the data to the byte code stack is the mechanism by which parameters are passed into the called byte code function. Likewise, return values can be copied from the byte code stack into the registers and/or native stack slots.
Generally, calling parameters can be either data (i.e., data parameters) or pointers to data (i.e., pointer parameters). However, increasingly, native code is using more sophisticated calling procedures such as, for example, calling a byte code function with a data calling parameter that is passed as data or is passed as a pointer to the data, depending upon the size of the data to be passed. For example, in an Intel® Extended Memory 64 Technology (a.k.a. EM64T) based platform, a data parameter with a data structure type (e.g., structure, union, class) will be passed either by value (through register or run-time stack) if the size of the data structure is ≦64 bits, or by pointer if the data structure size is >64 bits. If a pointer is passed, the pointer (64-bit) will be passed through register or run-time stack, while the actual data structure value (pointed by the pointer) exists in the run-time stack and/or any addressable memory location. Further, on an EM64T platform, the parameter passing mechanism is a dynamic one (i.e., either pass by value or pass by pointer) for a parameter with data structure type. Consider an example byte code function called from inside EM64T native code having the following function prototype:void foo (struct Q myParameter 1, struct T myParameter 2).Currently, the virtual machine has no way to know whether the example data structures are passed as data (i.e., by value) or a pointer to the data and, thus, the virtual machine cannot successfully implement the necessary thunking between the EM64T native code and the called byte code function. That is, the virtual machine will have no idea whether a 64-bit value that exists in a register (or a native stack slot) is an actual value of one of the data structure parameters or a pointer that points to an actual value. In such an example, it is impossible for the virtual machine to correctly copy the exact data structure value to the byte code stack.